By mounting a magnetic head (a skimming head) outside an insertion slot through which a card is to be inserted to illegally acquire the magnetic information, a fraudulent act may be performed on a magnetic recording medium processing device, which normally takes in a card (a magnetic recording medium) having magnetic information recorded thereon, transfers the card along a transfer passage and reads or writes the magnetic information with a magnetic head arranged on the transfer passage. As a measure to prevent such fraudulent acts, an interference field generating device is used to generate an interference magnetic field in the area outside the insertion slot to interfere with the illegal reading of the magnetic information.
For such an interference field generating device, a technology has been proposed for generating an interference magnetic field by providing and oscillating a resonance circuit in which a coil and a capacitor are connected in parallel (referring to Patent Reference 1). Another technology has also been proposed in which an H-bridge circuit with coil is configured by switches such as a transistor; when a separately provided capacitor and a coil, which are different from the ones of the H-bridge circuit, are electrically connected in series, the drive voltage and the voltage charged in the capacitor are applied to the coil (referring to Patent Reference 2).